1. Field of the Invention
The present invention relates to a thermostable photopolymeric film for liquid crystal (LC) alignment.
2. Description of the Prior Art
Several alignment materials have been known for homogeneous alignment of LC; polysiloxane polymers or polyamide polymers, in particular, are known to provide a high quality, thermostable alignment of various LC mixtures. To obtain a monodomain layer orientation for liquid crystal cells, several techniques are known: for example, stretching (H. Aoyama, Y. Yamazaki et al., Mol Cryst, Kiq. Cryst. Lett, 1981, Vol. 72, P 127), microgroove formation (E. S. Lee P. Vetter et al., Jpn Appl Phys., 1993, VOL. 32, P. L1436) and mechanical rubbing of the polymer surfaces (S. Kobayashi et al., Proc, SPIE, 1994, Vol. 2175, P 123; D. S. Soe et al., Mol, Cryst, Liq. Cryst., 1993, vol. 224, P. 13).
However, the above techniques have drawbacks. For example, microgrooves inherently contain defects and these defects are known to cause random phase distortions and light scattering, thus degrading display characteristics. Static electricity generated during the rubbing of the polymer surface is known to cause defects in active matrix LC displays. Moreover, it is practically impossible to orient selected regions of the LC surface locally so that each region has a different orientation with the above techniques.
Other techniques are also known. For example, W. M. Gibbons et al. Nature, 1991, vol. 351, p. 49, M Schdt et al., Jpn. J. Appl. Phys, 1992, vol. 31, part I, p. 2155, and T. Y. Marusii, Y. A. Reznikov. Mol. Mat., 1993, vol. 3, p. 161, EP 0525478 all describe methods of forming photopolymeric a orientation films. According to these other techniques, to form a photopolymeric orientation film, prepolymers were produced by the reaction of a polyvinylalcohol with substituted cinnamic acids. Then, the prepolymers were photopolymerized by irradiation with a linearly polarized ultra-violet (LTV) light beam.
The photopolymerization of the prepolymers caused by directed cross-linking of polyvinylcinnamates (PVCN) linear chains forms a new net polymeric structure with a higher order due to double bond opening reaction in cinnamoyl fragments, i.e. 2--2 cycloaddition reaction. Such a polymeric orientation film possesses an optical anisotropy and is known to planarly orient standard LC molecules in a preferred axial direction perpendicular to the polarization vector of the UV light beam.
However, the main disadvantage of such a photopolymeric orientation film is low thermostability, especially of oblique orientation. In addition, it has been experimentally discovered that with this type of polymer orientation films, the planar alignment of PVCN materials becomes thermally unstable at temperatures above 100-110.degree. C. Moreover, when the LC cell is heated above the clear point T.sub.1 and subsequently cooled, dramatic changes in the state of the liquid crystal cell (e.g., from isotropic to nematic to smectic to crystal) occur, degrading the quality of alignment. The thermostability of oblique alignment onto PVCN materials is very poor; it is not better than 50-60.degree. C. As the working temperature range of the LC cell tends to increase, the clear point of the LC cell consequently increases up to a temperature range of 80-100.degree. C. A conventional photopolymer film, which uses a PVCN as a prepolymer, cannot withstand such a high process and operating temperature range. The additional disadvantage of PVCN materials is a poor alignment quality of LC, particularly for active matrix LC displays.